A typical ratchet-type tensioner includes a plunger slidable in a plunger-accommodating hole in a tensioner housing. The plunger is biased in its advancing, i.e., protruding, direction, and abuts a flexible transmission medium directly, pr through a device such as a pivoted tensioner lever, to apply tension to the transmission medium.
A typical prior art ratchet-type tensioner 500 is disclosed in Japanese Utility Model No. 2559664 and illustrated in FIG. 15. In the tensioner, a plunger 514 is slidable in a plunger-accommodating hole in a housing 512, and protrudes from the housing, being biased in the advancing direction by a spring 518 and by oil pressure within a chamber 516 formed by the plunger and the housing.
A piston 526 slides in the housing 512 in a direction orthogonal to the direction in which the plunger 514 slides. An oil sub-chamber 520 is formed by the piston 526 and the housing 512, and an oil passage 544 supplies oil under pressure to the oil sub-chamber 520, urging the piston away from the plunger 514. A spring 534 biases the piston 526 toward plunger, opposing the force exerted by the oil in sub-chamber 520. Spring 534 is located within an air chamber 528 on the side of piston 526 opposite from the sub-chamber 520. An air hole 532 in communication with the air chamber 528 is closable by a rod 524, to which the piston 526 is attached, when the piston 526 is moved away from the plunger against the biasing force exerted by spring 534.
A rack of teeth 538 is provided on the plunger 514, and a plurality of teeth 536 capable of engaging the rack teeth 538 is provided at an end of rod 524 opposite form the end that is arranged to close off air hole 532. Surfaces of teeth 536 and 538 for blocking retraction of the plunger are formed at a right angle to a direction in which the plunger 514 moves.
In ratchet-type tensioners of the kind shown in FIG. 15, it is also known to provide cooperating surfaces for guiding the sliding movement of the ratchet element corresponding to rod 524 and piston 526.
If the plunger of the tensioner advances excessively when a timing chain becomes loose due to elongation as a result of fluctuations in tension or thermal expansion caused by changes in engine temperature, the restriction on retraction of the plunger due to operation of the ratchet mechanism can result in prolonged operation of the chain under excessive tension, which can result in noise as well as excessive forces that can shorten the useful life of the chain.
Japanese laid-open Patent Application No. 2004-36779 describes a tensioner designed to prevent a transmission chain from traveling under excessive tension. The tensioner has plunger having cut-away grooves and a spring-biased ball detent that engages a groove. The mechanism allows setting back, i.e., retraction, of the plunger when the chain is under excessive tension.
A ratchet mechanism that permits the plunger to set back when the tension of the chain is excessive can also be realized in a structure comprising rack teeth on the plunger, and cooperating teeth on a ratchet element that slides in a housing laterally relative to the direction of movement of the plunger. In this mechanism, the plunger is quickly set back by sliding of the ratchet element in a direction such that its teeth disengage the rack teeth, and excessive tension in the chain is reduced quickly. However, the reaction force imparted to the plunger as a result of excessive tension in the chain can cause the ratchet element to become inclined slightly relative to its sliding direction. When the ratchet element is inclined, both ends of the surface that is normally in sliding contact with a guide surface in the housing come into point contact with the guide surface. When point contact occurs, the slidability of the ratchet element relative to the guide surface is compromised, and the ratchet element no longer moves smoothly in the direction in which its teeth disengage the rack teeth.
In the tensioner shown in FIG. 15 and disclosed in Japanese Utility Model No. 2559664, the plunger is biased in the advancing direction by a plunger-biasing spring and by hydraulic pressure from oil supplied from an external source to the high-pressure oil chamber formed between the housing and the plunger. In this tensioner, there is also a problem that, on starting the engine, the hydraulic pressure in the tensioner can be insufficient to prevent setback of the plunger because of entry of air into the high pressure oil chamber while the engine is stopped. When the engine is started, the plunger is set back immediately by a reaction force from the chain, causing rattling sounds, referred to sometimes as “flapping noises,”